1. Field of the Invention
The present invention relates to a system for charging an energy reservoir, and to a method for operating the charging system.
2. Description of the Related Art
It is becoming apparent that in the future, both for stationary applications such as wind power installations and in vehicles such as hybrid or electric vehicles, increasing use will be made of electronic systems that combine new energy storage technologies with electrical drive technology. In conventional applications an electrical machine, which is embodied e.g. as a phase-sequence machine, is controlled via a converter in the form of an inverter. A characteristic of such systems is a so-called DC link circuit through which an energy reservoir, usually a battery, is connected to the DC voltage side of the inverter. In order to be able to meet the demands of a particular application in terms of power output and energy, multiple battery cells are connected in series. Because the current furnished by an energy reservoir of this kind must flow through all the battery cells, and because a battery cell can conduct only a limited current, battery cells are often additionally connected in parallel in order to increase the maximum current.
A series circuit of multiple battery cells yields not only a high total voltage but also the problem that the entire energy reservoir fails if a single battery cell fails, since battery current can then no longer flow. Such a failure of the energy reservoir can result in failure of the entire system. In a vehicle, a failure of the drive battery can leave the vehicle “stranded.” In other applications, for example rotor blade adjustment of wind power installations, unfavorable boundary conditions such as, for example, high wind can in fact lead to hazardous situations. A high level of reliability of the energy reservoir is therefore always desirable, “reliability” referring to the ability of a system to operate in fault-free fashion for a predetermined time.
German Patent Applications DE 10 2010 027857 and DE 10 2010 027861 describe batteries having multiple battery module sections that are connectable directly to an electrical machine. The battery module sections have a plurality of battery modules connected in series, each battery module having at least one battery cell and an associated controllable coupling unit that makes it possible, as a function of control signals, to interrupt the respective battery module section or bypass the respectively associated at least one battery cell or switch the respectively associated at least one battery cell into the respective battery module section. By appropriate application of control to the coupling units, e.g. with the aid of pulse width modulation, it is also possible to furnish suitable phase signals in order to control the electrical machine, so that a separate pulse width modulated inverter can be omitted. The pulse width modulated inverter required in order to control the electrical machine is thus, so to speak, integrated into the battery. For purposes of disclosure, these two earlier Applications are incorporated in their entirety into the present Application.
In contrast to conventional systems in which an electrical machine is controlled via an inverter and is supplied with electrical energy by an electrical energy reservoir separate therefrom, a constant DC voltage is not available in the case of the batteries described in the German Applications DE 10 2010 027857 and DE 10 2010 027861, so that such batteries cannot readily be integrated into conventional energy supply networks such as, for example, vehicle electrical systems of an electric or hybrid vehicle.